It is well known that using multiple-input multiple-output (MIMO) techniques in a wireless communication system significantly increases capacity in a multipath environment. However, multiple antennas increase complexity and cost because each transmit antenna and each receive antenna requires a separate radio-frequency (RF) chain including a modulator and/ or demodulator, AD/DA converter, up/down converter, and a power amplifier.
Antenna and/or beam selection can reduce the number of RF chains, while still taking advantage of the capacity and diversity increase provided by multiple antennas and/or beams. With antenna selection, each input/output RF chain is associated with one selected antenna. Antenna selection depends on small-scale fading, which varies with frequency. Therefore, an antenna selected for one frequency is usually not appropriate for some other frequency when the two frequencies are separated by more than one coherence bandwidth.
Beam selection depends on the path angles of arrival, which are approximately the same for the entire frequency band of interest. Beam selection associates each input/output RF chain with a selected beam, which can be formed by a linear transformation of the signal vector including the received/transmitted signals at all antennas.
In antenna and/or beam selection, typically, a channel submatrix is selected from a complete channel matrix, or a transformed channel matrix for beam selection, according to some criterion. To implement antenna and beam selection, the channel matrix is estimated by sending training frames that enable the two stations to estimate characteristics of the channel completely.
In the case both the stations have the selection capability, by reciprocity of the channel, the estimated channel should be the same in both directions, and both stations can select the same submatrix independently, without an explicit exchange of selection results. Then, the selected submatrix can be used for coherent detection of transmitted data frames.
However, because of channel ambiguity caused by estimation error or channel variation caused by time difference between training frames in different directions, the observed channel is different when a station is operating in transmit mode or receive mode, and the independent antenna selection can cause the two stations to select different submatrices. If different submatrices are used by the two stations, then the performance of the system can be degraded severely.
To solve this problem, explicit signaling can be used to exchange the information about the selection in either the physical (PHY) layer or the media access layer (MAC) layer of the stations. However, the additional signaling information in the physical (PHY) layer or signaling delay in the MAC layer is undesirable due to practical limitations.